In general, diverse circuits for performing diverse operations are provided inside of a semiconductor memory device, including a double data rate synchronous Dynamic Random Access Memory (DDR SDRAM) device. Among the circuits is a waveform generation circuit which generates a predetermined waveform. Examples of waveforms generated in a waveform generation circuit include sine waves, square waves, ramp waves, pyramidal waves, sawtooth waves and the like.
FIG. 1 is circuit diagram describing a general sawtooth wave generation circuit. Referring to FIG. 1, a sawtooth wave generation circuit includes a current source IS for generating a reference current I_REF, a capacitor C for being charged with the reference current I_REF, and an NMOS transistor NM for discharging capacitor C to a discharge terminal V_GND by being turned on in response to a pulse signal PUL.
FIG. 2 is a waveform diagram describing a circuit operation of the sawtooth wave generation circuit shown in FIG. 1.
Referring to FIGS. 1 and 2, a pulse signal PUL is a signal having a short pulse width and a predetermined period T. The NMOS transistor NM is turned on according to the period T. First, when the NMOS transistor NM is turned off, the capacitor C is charged with the reference current I_REF, and the slope of a sawtooth waveform signal S_OUT is determined based on the extent of charging. FIG. 2 shows a sawtooth waveform signal S_OUT generated when the waveform generation circuit of FIG. 1 operates as intended. Herein, the slope of the sawtooth waveform signal S_OUT is defined by the reference current I_REF and a capacitance of the capacitor C. Subsequently, when the NMOS transistor NM is turned on, the capacitor C is discharged to terminal V_GND.
However, it is substantially difficult to realize the desirable sawtooth waveform signal S_OUT in a conventional circuit structure. This is because the environmental factors reflected in the waveform generation circuit, such as, a process, a power source voltage, temperature and so forth, may be different, and these factors affect the slope of the sawtooth waveform signal S_OUT. For instance, when the amount of the reference current I_REF is changed according to the environmental factors reflected in a circuit, when the capacitance of the capacitor C is changed, when the size of parasitic capacitance of the NMOS transistor NM is changed, the target waveform and slope of a sawtooth waveform signal S_OUT may be changed. After all, the sawtooth waveform signal S_OUT does not maintain a uniform slope due to the environmental factors reflected in the circuit.
Sawtooth waveform signals S_OUT are usually used in circuits that are controlled in analog. In such circuits, the slope of the sawtooth waveform signals S_OUT is determined to achieve a particular result and when the waveform changes from the desired slope, the result cannot be achieved. Therefore, it is required to develop a circuit that can generate sawtooth waveform signals S_OUT having a uniform slope all the time in order to acquire the resultant object of a desired waveform.